<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <a href="http://www.baidu.com/">百度一下</a>
    <script>
        var a = document.querySelector('a')
        a.onclick = function(e){
            alert(1)
            return false
            // 兼容写法
            if(e.preventDefault){
                e.preventDefault()
            }else{
                return false;
            }
        }
    </script>
</body>
</html>